Global demand is rapidly rising for metals, especially rare metals, that are used in mobile telephones, personal computers and the like. However, price fluctuations of such metals can be rampant not just because of limited deposits and manufacturing quantities but also because of the unbalanced distribution of countries having such deposits and of political controls implemented on production quantity by countries with the resource. Securing a stable supply of rare metals has become extremely important. At the same time, the amount of rare metals that are used in mobile telephones, personal computers and the like that are discarded is also rapidly increasing. The recovery of rare metals from such waste has become an imperative. (Such waste that includes rare metals and precious metals is sometimes referred to as urban minerals.) However, since minerals and urban minerals that contain rare metals also contain many other metals, the process for the recovery of the desired metals (target metals) is complicated. No suitable recovery method has been available in the past, making the recovery cost high and the recovery effort to not make economic sense.
Among rare metals, much attention is now focused on lanthanoid elements, also referred to as rare earth elements, which exhibit special properties such as magnetism and light emission and make such elements indispensable for the high-tech technologies. “Lanthanoid element” is a collective term that refers to any of the 15 elements ranging from lanthanum with an atomic number of 57 to lutetium with an atomic number of 71. The special properties of lanthanoid elements derive from the fact that the outermost shell of the electron orbit is overlapped by other electrons and that the inner shell of the 4f electron orbit is filled with electrons. Because the electron orbit of the outermost shell is overlapped by other electrons, the chemical properties of any two lanthanoid elements are very similar, making it extremely difficult to separate two lanthanoid elements from each other. Natural minerals that contain lanthanoid elements often contain a plurality of different lanthanoid elements and actinoid elements as well. On the periodic table, actinoid elements belong to the same group 3 of elements as do the lanthanoid elements. Because the inner shells of actinoid elements are similarly filled with electrons, the chemical properties of any two actinoid elements are similar to each other, and the chemical properties of actinoid elements are also similar to those of lanthanoid elements. This makes it difficult to separate lanthanoid elements from actinoid elements when lanthanoid elements and actinoid elements coexist. Moreover, since actinoid elements include radioactive substances, it is indispensable when using lanthanoid elements that they be separated from actinoid elements, another factor that further complicates the extraction of lanthanoid elements. “Actinoid element” is a collective term that refers any of the 15 elements ranging from actinium with an atomic number of 89 to lawrencium with an atomic number of 103.
Currently, multi-stage extraction methods are used for the separation and recovery of lanthanoid elements and actinoid elements where slight differences in chemical properties to solvents are used in extraction processes involving the repetition of dozens to over 100 process steps to gradually increase the separation ratio. However, such multi-step extraction methods use large amounts of solvents and desorption agents, generate large amounts of waste solution and impose high processing costs and environmental costs.
In the field of elemental analysis, a field unrelated to methods of extraction of lanthanoid elements and actinoid elements, various research is under way looking into the use of mesoporous silica for the detection of metal ions. For example, Patent Literature 1 and 2 describe the use of hybrid sensors featuring colorant molecules such as aminoporphyrin, dithizone and porphyrin sulfone that are carried by mesoporous silica. Pb ions, Cd ions, mercury ions, Cr ions and the like are made to adsorb to the colorant molecules, and the changes in spectroscopic spectra are used to detect ion concentrations.